Temperature‐dependent transmission extended electron energy‐loss fine structure of aluminum

Inelastic electron scattering experiments in a transmission electron microscope provide a probe of core electron excitations that have binding energies below 2 keV, and that are localized within submicron diameter sample volumes. Extended electron energy‐loss fine‐structure measurements which show the variation with temperature of the mean squared relative displacement of aluminum yield a localized measurement of the Debye temperature which is in excellent agreement with macroscopic measurements

Kinetic, infrared, and X-ray absorption studies of adsorption, desorption, and reactions of thiophene on H-ZSM5 and Co/H-ZSM5

Temperature programmed desorption and infrared and X-ray absorption near-edge spectroscopies were used during adsorption and reactions of thiophene in order to probe adsorbed intermediates and catalytic structures responsible for thiophene reactions with propane or H2 on H-ZSM5 and Co/H-ZSM5.Infrared spectra showed that thiophene interacts with acidic OH groups in H-ZSM5 via hydrogen bonding at ambient temperature.No additional bands were detected on Co/H-ZSM5, suggesting the absence of specific interactions with Co cations. During adsorption at ambient temperatures, infrared bands assigned to CH2 groups near C=C bonds or Satoms and to S–H species were detected and H-ZSM5 and Co/H-ZSM5 acquired colors typical of thiophene oligomers.Slightly above ambient temperatures, benzene and H2S formed from pre-adsorbed thiophene.These results indicate that hydrogen-bonded thiophene undergoes ring opening or oligomerization near ambient temperature on acidic OH groups in H-ZSM5.Some of the adsorbed thiophene (20–50%) interacts weakly with channel walls or with residual Na cations and desorbs unreacted.The remaining adsorbed thiophene desorbs as H2S, aromatic hydrocarbons, and organosulfur compounds, such as methylthiophene and benzothiophene, or forms irreversibly adsorbed unsaturated organic deposits. In situ infrared studies during thiophene and thiophene–propane reactions at 773 K on H-ZSM5 and Co/H-ZSM5 showed that surface coverages of thiophene-derived intermediates were low on acidic OH groups and Co cations.Co K-edge X-ray absorption near-edge spectra measured during these reactions confirmed that Co2+ cations do not reduce or sulfide; their local environment, however, changes slightly, apparently because of interactions of strongly adsorbed species with Co cations.Sulfur K-edge X-ray absorption spectra detected small amounts of organosulfur species, but no inorganic sulfides, after thiophene, thiophene–H2 , and thiophene–propane reactions, consistent with the observed stability of exchanged cations against reduction and sulfidation.S : Al ratios were less than 0.04 at. on all samples; these amounts represent less than 1% of the S-atoms removed from thiophene as H2S during catalytic propane–thiophene reactions.National Science Foundation (CTS-96-13632

Synthesis, structural characterization, and catalytic properties of tungsten-exchanged H-ZSM5

W-exchanged H-ZSM5 was prepared by sublimation of WCl6 at 673 K followed by hydrolysis of exchanged WClx species at 523 K. D2 exchange with residual OH groups showed that each W initially replaced about two zeolitic protons for W/Al ratios of 0.29 and 0.44, consistent with the formation of (WO2)2+ containing W6+ species bridging two cation exchange sites. As temperatures reached973 K during D2-OH exchange, these species reduced to (WO2)+ with the concurrent formation of one OD group. CH4 conversion turnover rates (per W) and C2-C1 2 selectivities are very similar to those observed on a Mo/H-ZSM5 sample with similar cation exchange level. As in the case of Mo/H-ZSM5, WOx/H-ZSM5 precursors are initially inactive in CH4 reactions, but they activate during induction with the concurrent evolution of CO, H2O, and an excess amount of H2. The reduction and carburization processes occurring during CH4 reactions and the structure of the exchanged WOx precursors was probed using in situ X-ray absorption spectroscopy (XAS). XAS studies confirmed the isolated initial nature of the exchanged WOx precursors after hydrolysis and dehydration and the formation of WCx clusters 0.6 nm in diameter during CH4 reactions at 973 K. The structural and catalytic resemblance between W- and Mo-exchanged H-ZSM5 is not unexpected, in view of chemical similarities between oxides or carbides of Mo and W. The synthesis of exchanged WOx precursors and their subsequent carburization during CH4 reactions, however, are more difficult than the corresponding processes for the MoOx counterparts. This may account for previous reports of lower CH4 reaction rates and aromatics selectivities on W/H-ZSM5 compared with those observed on Mo/H-ZSM5 and with those reported here for rigorously exchanged W/H-ZSM5